can I replace the 15v fuse for the "power outlet" in my 1994 cav with a 12v fuse. The heater doesn't work anymore and I want to use a "plug in" heater that runs on 12v

All modern car fuses are 12volts, if you mean change the fuse to a higher amp, no no it will burn up something, it seems like you are trying to use an electric space heater, no way...there are converters that will allow you to plug a household 120V plug in to the power outlet but they will not feed much..I use one for a cell phone charger since I don't have a regular car charger, but that is about all the will handle. a bit more than a cell phone but not a whole lot

But..if you have an electric 12volt space heater, fine, I see what you mean there, but 1) do not raise the amperage of the fuse 2) please be careful where you put that heater, and it will be fine as long as it draws no more than 15 amps

Short answer - replace it. --- Fuses are short circuit protection the protect the wires and prevent fires. They are not all that good for protecting from overloads. A 12 volt and 15 volt fuse are essentially the same. It's the amount of amperes that are allowed to pass that makes the difference.